Field of the Invention
The present invention is related to an uninterruptible power supply that supplies power to an external device from a built-in battery in case of abnormality of a commercial power supply, and particularly to an uninterruptible power supply that can extend the lifetime of a built-in battery and to a control method of the uninterruptible power supply.
Description of Related Art
Usually, an uninterruptible power supply (UPS) is used by being connected between a commercial power supply and an external device (for example. an electric device such as a server, a personal computer, etc.).
An uninterruptible power supply outputs power of a commercial power supply to an external device when the commercial power supply is normal, and charges a built-in battery with power obtained from the commercial power supply.
When a power failure occurring on the commercial power supply side prevents the supply of power from a commercial power supply, the uninterruptible power supply supplies power accumulated in a built-in battery to an external device.
Also, in addition to the above measures against power failures, an uninterruptible power supply is used when peak-cut or peak-shift measures of power are taken.
In an uninterruptible power supply described in Japanese Laid-open Patent Publication No. 2009-140507 (hereinafter “JP'507”; see, e.g., FIG. 13), a configuration is employed in which a plurality of uninterruptible power supplies are connected with the same power line and power of a battery is supplied through the power line from an uninterruptible power supply having a sufficient remaining capacity to a different uninterruptible power supply having a smaller battery capacity.
FIG. 1 shows a configuration of a conventional uninterruptible power supply disclosed in JP'507. In FIG. 1, UPSs (uninterruptible power supplies) 2-1, . . . , 2-n employ a configuration in which power from an AC power supply is supplied to a corresponding PC (Personal Computer) 11 so that a built-in battery is charged with power from an AC power supply when an AC power supply is normal, and power accumulated in this battery is supplied to the corresponding PC 11 in case of abnormality (power failure etc.) of an AC power supply. Also, the UPSs 2-1, . . . , 2-n are connected to each other through an inter-UPS power supply line and are connected through an inter-UPS control signal line.
Explanations will be given for a case when the UPS 2-1 whose battery capacity remains sufficient supplies battery power to a different UPS 2-n having insufficient battery capacity. In such a case, a power transmission control unit (not shown) of the UPS 2-1 supplies its battery power to the inter-UPS power supply line. Upon this, the power transmission control unit (not shown) on the UPS 2-n side from among other UPSs 2-2, . . . , 2-n is set to be in a state in which it can receive power on the inter-UPS power supply line, and other UPSs 2-2, . . . , 2-n−1, etc. are set to be in a state in which they cannot receive power on the inter-UPS power supply line. In this state, the battery power from the UPS 2-1 will be supplied only to the UPS 2-n. 
However, there has been a problem in which accommodation of power between a battery having a sufficient remaining battery capacity and a battery having an insufficient remaining battery capacity results in a situation where sufficient power cannot be supplied to an external device in case of power failures etc.